5441336927

Cherutich

70

Report 8

Pressure (bar)

FIGURĘ 4: Pressure logs for a few Olkaria wells

4. COMPARISON OF TOP DRIVE AND ROTARY TABLE TYPES OF RIGS

Drilling is considered to be an industry where the finał product is a borehole. Geothermal drilling technology is borrowed from the petroleum industry' with some modification to fit the geothermal conditions of high temperaturę and pressure, hard formation and large diameter casing design. In Iceland, before 1958 the rigs being used could only drill wells to less than 250 m depth, but with the arrival of “The Steam Rig” in 1958 the drilling of deep high-temperature wells to morę than 500 m began. With advancements in technology, the rig industry now offers a selection of modem electric or hydraulic rigs with different degrees of automation for the handling of drill pipes and casing. The successful completion of a geothermal well on time and with a good safety margin depends on the efficiency of the rig and the level of skill of the rig crew members plus the proper planning and coordination of all the pre-drilling activities like civil works, procurement of spare parts and consumables reąuired in the drilling operation. The first step is choosing the right type and size of rig w ith a good safety margin that can fit into the wellbore planning design.

4.1 The drive mechanism of a rig

The main difference between the top drive rig and the convection system rig is the position of the drive mechanism. The top drive system is situated on the mast and moves w ith the drill string up and down along the mast guide. The top drive system torąue comes from a hydraulic or electrical motor drive. The rotary table drive system is situated on the rig floor. The rotary table drive system is mechanical where the torąue is transmitted to a hexagonal pipę called the Kelly, through a chain and a sprocket.